The invention relates to a method and an apparatus for producing a persistence trail for at least one target whose respective current position is represented in a continuous manner in a radar image of a radar instrument.
Such persistence trails which are produced on the radar screen in a manner assigned to individual targets describe the tracks of located targets on the radar image, from which an observer can distinguish the movement behavior of the targets and also recognize collision risks evident from the directions of movement and eliminate them by suitable maneuvers instigated by said observer.
In a known method of this type (DE 29 24 176 C2), the radar echoes of the current panorama sample and of a plurality of preceding panorama samples, after storage in a single image memory, are displayed on the screen of the radar instrument. The image memory used is a read/write memory which is oriented in x,y coordinates and in which storage is performed by storing the intensities of the radar echoes currently received in each case in successive samples of the panorama, with the memory addresses corresponding to their spatial coordinates. The intensities obtained from previous samples are preserved in this case. The image content of the image memory is read out independently of the chronological order of reception of the radar echoes and represented on the radar image. As a result, the intensities stored for successive panorama samples appear strung together on the screen and describe the tracks or the persistence trails of the targets.
In a further configuration of the known method, the stored intensities are reduced by one or more increments after a predeterminable number of panorama samples, as a result of which the preceding target positions are represented on the screen with an intensity that decreases with age. The length of the persistence trails is thus dependent not only on the target speed but also on the speed with which a complete panorama sample is effected, and thus allows the time interval for which the persistence trail is represented to be assigned to the length of the persistence trail only in the case of a constant rotational speed of the radar antenna. If the rotational speed is changed, the time interval must be redefined.
The invention is based on the object of improving a method of the type mentioned in the introduction to the effect that the persistence trails, with little outlay, appear in the radar image in a manner that is highly visible to the observer without overburdening the radar image in the case of a plurality of simultaneously represented targets.
The object is achieved according to the invention by means of the features in patent claim 1.
The method according to the invention has the advantage that it can be carried out using standard graphics cards in a standard screen or video device and, as a result, can be implemented practically without any additional outlay. The persistence trails visible on the screen are very clear and do not overburden or overload the radar image even in the case of a multiplicity of targets appearing on the radar image, since, by virtue of the thinning out of pixels that increases with increasing age, their luminosity diminishes towards that end of the persistence trail which faces away from the current target position.
Expedient embodiments of the method according to the invention with advantageous developments and configurations of the invention emerge from the further claims.
The method according to the invention can be realized in a particularly advantageous manner in that, in accordance with a preferred embodiment of the method, the amplitudes of the radar echoes received by the radar instrument are stored in a radar video memory, whose memory cells are oriented in x,y coordinates, in that the memory content of the radar video memory is written as 1-bit information memory cell by memory cell to at least one trail memory, whose memory cells are oriented in x,y coordinates, in so far as the memory content of the individual memory cells exceeds a first predetermined value, in that the memory content of the trail memory is continuously erased according to an e.g. pseudo-random method for thinning out pixels, in that firstly the memory content of the trail memory is written to an image buffer memory, whose memory cells are oriented in x,y coordinates, and, chronologically succeeding that, the memory content of the radar video memory is written, in so far as the memory contents of the individual memory cells of the radar video memory exceed a second predetermined value, and in that the memory content of the image buffer memory is represented in a continuous manner on the radar screenxe2x80x94oriented in x,y coordinatesxe2x80x94of the radar instrument as a pixel per memory cell.
Storing the persistence trails in a special trail memory not only enables the pixels to be thinned out in a particularly simple manner, but also enables, without difficulty, the changeover of the representation mode true-motion to the representation modes relative-motion and relative-motion true trails (center display), in which one""s own location, that is to say one""s own ship carrying the radar instrument, is fixed in the radar image, preferably in the center thereof, and vice versa, since only the memory content of the trail memory has to be cyclically shifted in accordance with the movement of one""s own ship or the addressing of the memory cells of the trail memory has to be cyclically changed in accordance with the movement of one""s own ship. If two trail memories are used, no loss of the persistence trails is associated with the changeover, with the result that, unlike in the case of known radar instruments, it is not necessary to accept a waiting time, which lasts at least a plurality of antenna rotations, until the persistence trails reappear on the radar image in the representation mode respectively chosen.
The provision of the additional memories having the same memory size has hardly any appreciable effect on the production costs of a screen device operating according to the method, since memory cards of the required size are extremely inexpensive. Furthermore, a color coding can be achieved in a simple manner using the trail memory, with the result that the persistence trails can also additionally be represented in different colors.